Testing of elastomeric materials

ABSTRACT

Wall means surrounds and defines an interior chamber in which a sample of vulcanized elastomer is to be accommodated. At least two separate surfaces bound this chamber and are provided with undercut grooves of dovetail-shaped cross-section in which portions of the sample are received. At least one surface can be moved with reference to the other in a sense effecting distending of the test sample.

muted States atent 1 [111 emes Wolff et al. H lMarch 13, 1973 TESTING OF ELASTOMERIC MATERIALS [56] References Cited [75] Inventors: Siegfried Wolff, Bezirk Cologne; UNITED STATES PATENTS Siegfried Baumgart, Hurth Bezirk Cologne; Ulf-Erick Arnold, Hurth- 2,679,663 6/1954 Schwemler et al. ..18/38 Hannelore Pohnisch Erfgtadt. Beatty et a1. Dirmerzheim; peter Herbrich 3,538,758 11/1970 Karper et al .73/l0l Cologne, all of Germany Primary ExaminerJerry W. Myracle [73] Assignee: Deutsche Gold-Und Sllber-Scheide- Att0mey Michael S Striker anstalt Vormals Roessler, Frankfurt 7 am Main, Germany [57] ABSTRACT [22.] plied: 1971 Wall means surrounds and defines an interior chamber [21] Appl. No.: 106,129 in which a sample of vulcanized elastomer is to be accommodated. At least two separate surfaces bound [30] Foreign Application Priority Data this chamber and are provided with undercut grooves of dovetail-shaped cross-section in which portions of Jan. 15, 1970 Germany ..P 20 01 613.1 the Sample are received. At least one Surface can be moved with reference to the other in a sense effecting [52] U.S. Cl ..73/101 distending f h test sample [51] Int. Cl. ..G01n 3/32 [58] Field of Search ..73/l0l, 15.6; 18/38 7 Claims, 9 Drawing Figures PATENTEDMARI 3mm I 7 0,099

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TESTING OF ELASTOMERIC MATERIALS BACKGROUND OF THE INVENTION The present invention relates to the testing of vulcanized elastomers. More particularly, the invention relates to an apparatus for such testing and to a testing method.

The vulcanization of the elastomeric materials must often be tested to observe its progress and the behavior of the material at different vulcanizing stages. Such tests are carried out in so-called Vulkameters which are known to those skilled in the art and which, basically, operate on one of two principles: either by measuring torque (torsional-thrust Vulkameters) or by measuring thrust (linear-thrust Vulkameters). These devices are constructed and operate analogously to the shear-disk plastometer of Mooney which is described in German Industrial Norm DIN 52,523, or the AGFA Vulkameter which is described in Kautschuk und Gummi, (1957), pages 168-172.

Generally speaking, the test sample is accommodated in a chamber and is subjected therein to distending or deforming. in the case of torsional'thrust Vulkameters this is accomplished by a substantially mushroom-shaped rotor which extends into the chamber into contact with the sample and carries out a periodically changing rotation; in the case of linearthrust Vulkameters the chamber is provided in a twosection housing and one section is displaced with reference to the other.

To obtain measurements or test results which properly and completely reflect the progress of the vulcanizing process, it is essential that the test sample adhere properly and constantly to the surfaces with which it is in engagement, that is those bounding the chamber and, in case a separate rotor element is involved which acts upon the sample, also the surface of the rotor element. However, as a general rule the samples will shrink during vulcanization, a factor which can be proven by measuring the density of the sample during the vulcanizing process. Such measurements as a rule indicate an increase in density which can only be the result of a decrease in volume, i.e. shrinkage. Such shrinkage, however, causes the sample to recede from the requisite intimate and constant contact with the aforementioned surfaces bounding the chamber, leading to inaccurate test results.

Attempts have been made to overcome this problem, in the prior art. Thus, a torsional-thrust Vulkameter is described in ASTM Special Technical Publication No. 383, Philadelphia, Pa. April 1965, pages 51-75, where a feature is incorporated which is intended to correct the difficulty. in this device a two-section housing. is employed the upper section of which is provided with a passive diaphragm. This diaphragm has a small inherent elasticity and is intended to become resiliently deflected when the chamber-filled with the test sampleis closed so that the test sample presses against the diaphragm, and vice versa. On the other hand, when the sample shrinks, the diaphragm is intended to continue to exert some pressure upon it due to the tendency on the part of the diaphragm to return to its undeflected rest position. Theoretically, this will maintain the sample in intimate engagement with the requisite surfaces of the testing apparatus; in practice, however, it has been found that this desired result is not always reliably achieved.

Another prior-art approach suggests adjusting the interior pressure in the test chamber via an actively adjustable diaphragm or an analogously acting component. This involves constant measuring of the interior pressure by a pressure-sensitive instrumentality, and comparison of the measured result with an established pressure factor; the difference between the measured pressure and the established factor serves as the basis for regulating the actual pressure in the interior of the chamber. However, this approach is not only very complicated and hence subject to malfunction as well as being expensive; it, also, does not entirely preclude the development of the earlier-discussed undesirable conditions.

Finally, the prior art also suggests that the surfaces to which the sample is intended tointimately adhere, be provided with grooves or rectangular or rounded crosssection. Observation has shown that, although this measure somewhat retards the receding of the sample out of contact with the surfaces in question during shrinkage of the sample, it is not capable of preventing the final receding and separation so that the undesired conditions will occur, even through somewhat later than would otherwise be the case.

SUMMARY OF THE INVENTION It is, accordingly, an object of the present invention to completely prevent the occurrence of the aforementioned undesired conditions.

More especially, it is an object of the present invention to provide an improved method of testing vulcanized elastomeric samples, in which these undesired conditions cannot occur and which, therefore, yields test results which are not in any way influenced by such conditions.

A concomitant object of the invention is to provide an improved testing apparatus for carrying out the novel method.

In pursuance of the above objects, and of others which will become apparent hereafter, one feature of the invention resides in an apparatus for testing of vulcanizable elastomers which, briefly stated, comprises wall means having exposed surfaces which define with one another an interior chamber adapted to accommodate an elastomeric sample to be tested, at least one of these surfaces being movable with reference to the other. The surfaces are provided, in accordance with the present invention, with recesses of undercut crosssection, and portions of the sample are accommodated in these recesses. Thus, when the material of the elastomeric sample shrinks during vulcanizing and attempts to recede inwardly away from the surfaces in question, the portions accommodated in the undercut recesses will be prevented from such receding as soon as after a small initial amount of receding they are stopped from further movement of this type by contact with the faces bounding the undercut recesses.

The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a diagrammatic illustration of a prior-art torsional-thrust Vulkameter;

FIG. 2 is a view similar to FIG. 1 but of a prior-art linear-thrust Vulkameter;

FIG. 3 is a fragmentary sectioned detail view, on an enlarged scale, showing another prior-art embodiment;

FIG. 4 is a view similar to FIG. 3 but showing still a further prior-art embodiment;

FIG. 5 is a graph showing a test result achieved in the apparatus of FIG. 1;

FIG. 6 is a view similar to FIG. 5 but showing a test result obtained using the apparatus of FIG. 3 or 4;

FIG. 7 is a view similar to FIGS. 3 and 4, but showing an apparatus according to the present invention;

FIG. 8 is a view similar to FIGS. 5 and 6 but showing a graph of a test result obtained using the apparatus according to the present invention; and

FIG. 9 is a graph comparing the test results shown in FIGS. 5, 6 and 8.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Discussing now the drawing in detail, and firstly the prior art apparatus in FIG. 1, it will be seen that this comprises a housing having two sections 1 and 2 which are provided with hollows facing one another and together defining the interior chamber 3. The sections 1 and 2 can be moved apart and together in the sense indicated by arrow 4 to permit access to the chamber 3.

A rotor 6 extends with its substantially mushroomshaped head into the interior of chamber 3 through an opening provided for this purpose in the section 2. Rotor 6 is turnable in the sense indicated by the curved arrow. Its surface, and the surfaces bounding the hollows in the sections 1 and 2, together define the testing chamber 3 in which the sample 5 of vulcanizable elastomeric material is accommodated for testing purposes.

It is evident that when shrinkage of the sample 5 occurs, there is nothing to prevent the sample from receding out of contact-or at least out of intimate, constant and non-slip contact-with the surfaces of the sections 1 and 2 and ofthe rotor 6.

FIG. 5 is a graph showing the test result achieved with the apparatus of FIG. 1. Explanations concerning the graph are given on the graph shown in FIG. 6.

Whereas FIG. 1 shows a torsional-thrust Vulkameter, FIG. 2 shows a linear-thrust Vulkameter. The same reference numerals have been used in FIG. 2 to indicate components similar to those of FIG. 1. It is selfevident that, although the operation here is somewhat different in that the section 1 moves in the sense of arrow 4a to obtain the test results, there is nothing to prevent the receding of the sample 5 in the same manner as in FIG. 1.

FIG. 3 shows a portion of one of the surfaces bounding the chamber 3, namely one of the surfaces bounding the hollows in sections 1 and 2 or of the outer surface of the head of rotor 6. The prior art provides, according to the embodiment shown in FIG. 3, that any and all of these surfaces may be provided with grooves 7 of rectangular cross-section. This improves adherence of the sample 5 to the respective surfaces. FIG.

4 differs from FIG. 3 only in that the grooves 8 may be of rounded or semi-circular cross-section as shown.

Results of a test made with apparatus using either the grooves 7 of FIG. 3 or the grooves 8 of FIG. 4, are shown in the graph of FIG. 6 which is self-explanatory.

Thus far, the prior art has been discussed, Coming now to the present invention it is pointed out that the apparatus a fragment of which is shown in FIG. 7, may be of either the type shown in FIG. 1 or the type shown in FIG. 2. However, in accordance with the present invention and as shown in FIG. 7, we provide the surfaces bounding the chamber 3 with recesses-ordinarily grooves-9 which are of undercut cross-sectional configuration. More especially, in the embodiment of FIG. 6, the grooves 9 are of dovetail-shaped undercut crosssection. When portions of the sample 5 are received in such recesses 9, and when shrinkage occurs, such portions can recede only to the extent and in the manner indicated by the broken line 50, namely to some extent away from the transverse bottom face 9a of the respective recess 9. In so doing, however, the material of the shrinking portion of the sample 5 retracts in the direction of the three arrows shown in FIG. 6, and thus firmly adheres against the side faces 9b which prevent it from receding any further. Proper contact with the respective surface is therefore retained under all circumstances. Measuring means MM has been shown diagrammatically, for measuring the physical characteristics of the sample which are to be determined; such measuring means is known per se and requires no detailed description.

Tests have shown that it is most advantageous if the side faces include with the surface la and/or the transverse face 9 an angle of at least substantially 45. It is further advantageous not to provide all of the grooves in the respective surface bounding the chamber 3 as undercut grooves 9; instead, the grooves 9 should preferably alternate with grooves 7 of rectangular cross-section or with grooves 8 of rounded cross-section, such as are shown in FIGS. 3 and 4, respectively. As shown in FIG. 6 it is possible to use grooves 7 and 8 together, but either the grooves 7 or the grooves 8 may be used by themselvesalways in conjunction with grooves 9, of course.

FIG. 8 in a graph showing test results obtained with an apparatus according to the present invention, that is an apparatus of the type shown in FIG. 1 or in FIG. 2, but constructed in accordance with the present inventive concept as shown and described with reference to FIG. 6. The graph in FIG. 8 shows a vulcanizing isothermal, with the sample tested being taken from an SBR-ISOO mixture. A comparison with FIGS. 5 and 6 clearly indicates that in FIG. 8 the value in the terminal reaction period-measured between points D and E- is substantially better than in FIGS. 5 and 6. In FIG. 8 the curve remains steady, whereas in FIGS. 5 and 6 it declines. In FIG. 5, in fact, a decline of the torque peak takes place much earlier, so that the further progress of the curve (to the area between points D and E) is no longer characteristic.

Reference to FIG. 9 will illustrate this point. The material characteristics of the sample are observed in the torsional-thrust Vulkameter to the optimum vulcanization point. This is represented by curve I in FIG. 9. Such an observation is not possible in the Mooney device mentioned earlier, because the material of the sample recedes from the rotor of the device upon reaching a certain stage of vulcanization. This is indicated by the curve 2 of FIG. 9.

Extensive tests and examinations have shown that the optimum vulcanization value (measured between points D and E on the graphs of FIGS. 5, 6 and 8) can not be measured with absolute reliability in torsionalthrust Vulkameters. Such an observation is possible, for instance, in the case of natural rubber-dicumylperoxide mixtures with or without carbon-black filler (and is represented by curve 1 in FIG. 9). On the other hand, if the mixture is one of the widely used synthetic rubber-sulphur mixtures, a decline in the torque peak prior to reaching of the optimum vulcanization value is observed and is frequently indicated by a fault line U in the curve recorded by the torque recorder (see curve 3 in FIG. 9 and see also the graph in FIG. 5). Quite often the decline is also constant as indicated by curve 4 in FIG. 9 and shown in the graph of FIG. 6. The latter possibility is especially bothersome because the viewer cannot readily detect that the curve is incorrect.

The reason for these problems is the aforediscussed receding of the sample from the associated surfaces of the testing chamber. The prior-art attempt at counteracting the problem with the use of a pressure-adjusting diaphragm is indicated at V in curve 3 of FIG. 9.

The present invention, however, completely avoids such problems in the manner and for the reasons discussed earlier, so that the curve shown in the graph of FIG. 8 is always obtained. Thus, the decline in torque peak observed in torsional-thrust Vulkameters is completely eliminated, a significant increase in the accuracy and reliability of 'test results is obtained, and the test results are reproducible from case to case with a reliability not heretofore extant. Furthermore, the present invention is applicable also when a required precise temperature control makes it necessary that the distance between sample and point of temperature regulation be as small as possible.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of applications differing from the types described above.

While the invention has been illustrated and described as embodied in an apparatus for testing of vulcanizable elastomers, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt if for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention, and therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

1. In an apparatus for testing physical characteristics of vulcanizable elastomers, in combination, wall means having exposed surfaces which define with one another an interior chamber adapted to accommodate an elastomerrc sample to be tested, at least one of said surfaces being movable with reference to the other; a plurality of recesses of undercut cross-section provided in said surfaces for accommodating portions of said sample so as to prevent separation of the latter from said surfaces; and measuring means for measuring a physical characteristic of said sample while the latter is accommodated in said interior chamber.

2. In an apparatus as defined in claim 1, wherein said undercut recesses are of substantially dovetail-shaped cross-section.

3. In an apparatus as defined in claim 2, said undercut recesses each being bounded by a transverse surface portion inwardly spaced from the respective surface, and at least a pair of lateral surface portions extending from said transverse surface portion to the respective surface and including with at least one thereof an angle of at least substantially 45.

4. In an apparatus as defined in claim 2, wherein said recesses are elongated grooves.

5. In an apparatus as defined in claim 1; and further comprising additional recesses of other-than-undercut cross-section provided in said surfaces and each located between two adjacent recesses of undercut cross-section.

6. In an apparatus as defined in claim 5, wherein said additional recesses are of substantially semi-circular cross-section.

7. In an apparatus as defined in claim 5, wherein said additional recesses are of substantially rectangular cross-section. 

1. In an apparatus for testing physical characteristics of vulcanizable elastomers, in combination, wall means having exposed surfaces which define with one another an interior chamber adapted to accommodate an elastomeric sample to be tested, at least one of said surfaces being movable with reference to the other; a plurality of recesses of undercut cross-section provided in said surfaces for accommodating portions of said sample so as to prevent separation of the latter from said surfaces; and measuring means for measuring a physical characteristic of said sample while the latter is accommodated in said interior chamber.
 1. In an apparatus for testing physical characteristics of vulcanizable elastomers, in combination, wall means having exposed surfaces which define with one another an interior chamber adapted to accommodate an elastomeric sample to be tested, at least one of said surfaces being movable with reference to the other; a plurality of recesses of undercut cross-section provided in said surfaces for accommodating portions of said sample so as to prevent separation of the latter from said surfaces; and measuring means for measuring a physical characteristic of said sample while the latter is accommodated in said interior chamber.
 2. In an apparatus as defined in claim 1, wherein said undercut recesses are of substantially dovetail-shaped cross-section.
 3. In an apparatus as defined in claim 2, said undercut recesses each being bounded by a transverse surface portion inwardly spaced from the respective surface, and at least a pair of lateral surface portions extending from said transverse surface portion to the respective surface and including with at least one thereof an angle of at least substantially 45*.
 4. In an apparatus as defined in claim 2, wherein said recesses are elongated grooves.
 5. In an apparatus as defined in claim 1; and further comprising additional recesses of other-than-undercut cross-section provided in said surfaces and each located between two adjacent recesses of undercut cross-section.
 6. In an apparatus as defined in claim 5, wherein said additional recesses are of substantially semi-circular cross-section. 